Set defaults for high-pri and low-pri thread pools in regression test script

Summary:
**Summary**:
Set defaults for high-pri and low-pri thread pools in regression test script.

**Reason for this change**:
With #2680 , high-pri and low-pri thread pools get different numbers than before if  `num_high_pri_threads` and `num_low_pri_threads` options are not explicitly passed to db_bench in regression test script ... leading to a false-positive regression.

**Test Plan**:
REMOTE_HOST=udb1671.prn3 TEST_MODE=1 FBSOURCE=~/fbsource ~/fbsource/fbcode/rocks/tools/debug_regression_test.sh viewstate  (with very minor changes to the internals).

Observe P50 and P99 which showed up as regressions in our graphs.

Stats with the commit prior to #2680 , ie. 4f81ab3 :
seekrandomwhilewriting :      75.096 micros/op 13316 ops/sec;  168.6 MB/s (7499074 of 7500000 found)
Microseconds per seek:
Count: 120000000 Average: 1197.7254  StdDev: 33.35
Min: 187  Median: 980.5292  Max: 1816424
Percentiles: **P50: 980.53** P75: 1494.57 **P99: 4185.64** P99.9: 7800.11 P99.99: 15039.64

Stats at #2680, ie. at commit dce6d5a (false-positive regression):
seekrandomwhilewriting :      85.330 micros/op 11719 ops/sec;  148.4 MB/s (7499073 of 7500000 found)
Microseconds per seek:
Count: 120000000 Average: 1362.3261  StdDev: 27.86
Min: 185  Median: 1088.1915  Max: 652760
Percentiles: **P50: 1088.19** P75: 1658.12 **P99: 5361.15** P99.9: 7997.95 P99.99: 11730.07

Stats with the current change on top of dce6d5a :
seekrandomwhilewriting :      77.780 micros/op 12856 ops/sec;  162.8 MB/s (7499102 of 7500000 found)
Microseconds per seek:
Count: 120000000 Average: 1226.6744  StdDev: 17.16
Min: 185  Median: 994.2956  Max: 2553530
Percentiles: **P50: 994.30** P75: 1513.68 **P99: 4284.30** P99.9: 9338.64 P99.99: 23008.86
Closes https://github.com/facebook/rocksdb/pull/2801

Differential Revision: D5742338

Pulled By: sagar0

fbshipit-source-id: cc5d727c1a131f2a7070d1bb892efbe929b976ff

tools/regression_test.sh

@@ -85,6 +85,10 @@
 #       db_bench.  Default: 4.
 #   MAX_BACKGROUND_COMPACTIONS:  The maximum number of concurrent compactions
 #       in db_bench.  Default: 16.
+#   NUM_HIGH_PRI_THREADS:  The number of high-pri threads available for
+#       concurrent flushes in db_bench.  Default: 4.
+#   NUM_LOW_PRI_THREADS:  The number of low-pri threads available for
+#       concurrent compactions in db_bench.  Default: 16.
 #   SEEK_NEXTS:  Controls how many Next() will be called after seek.
 #       Default: 10.
 #   SEED:  random seed that controls the randomness of the benchmark.
@@ -182,6 +186,8 @@ function init_arguments {
   STATS_INTERVAL_SECONDS=${STATS_INTERVAL_SECONDS:-600}
   MAX_BACKGROUND_FLUSHES=${MAX_BACKGROUND_FLUSHES:-4}
   MAX_BACKGROUND_COMPACTIONS=${MAX_BACKGROUND_COMPACTIONS:-16}
+  NUM_HIGH_PRI_THREADS=${NUM_HIGH_PRI_THREADS:-4}
+  NUM_LOW_PRI_THREADS=${NUM_LOW_PRI_THREADS:-16}
   DELETE_TEST_PATH=${DELETE_TEST_PATH:-0}
   SEEK_NEXTS=${SEEK_NEXTS:-10}
   SEED=${SEED:-$( date +%s )}
@@ -231,6 +237,8 @@ function run_db_bench {
       --max_background_flushes=$MAX_BACKGROUND_FLUSHES \
       --num_multi_db=$NUM_MULTI_DB \
       --max_background_compactions=$MAX_BACKGROUND_COMPACTIONS \
+      --num_high_pri_threads=$NUM_HIGH_PRI_THREADS \
+      --num_low_pri_threads=$NUM_LOW_PRI_THREADS \
       --seed=$SEED) 2>&1"
   ps_cmd="ps aux"
   if ! [ -z "$REMOTE_USER_AT_HOST" ]; then